1. Field of the Invention
The invention relates to a thermosensitive recording material including a substrate, a thermosensitive recording layer containing color formers and color acceptors, and a protective layer formed on the recording layer; and use thereof as a ticket.
2. Description of the Related Art
A thermosensitive recording sheet is disclosed in EP-B-0 909 242. The protective layer of the recording sheet which is to be used as a label, cash register receipt or ticket, and which can be readily printed on by the wet offset process, includes in particular, bentonite as a pigment, with a pigment content of from 20 to 40% by weight, a binder content which includes relatively large proportions of water-insoluble binder in the form of a polymer dispersions and relatively small proportions of an aqueous solution of a polymer. This publication discloses natural calcium carbonate, precipitated calcium carbonate, silica, alumina and titanium oxide as further pigments in the protective layer. While polyvinyl alcohol is mentioned as the preferred water-soluble binder, the water-insoluble binders are preferably self-crosslinking copolymers of acrylonitrile, methacrylamide and acrylic esters.
EP-A-0 344 705 discloses a protective layer in which from 0.5 to 3 parts by weight of pigment are present per part by weight of binder. Inter alia, polyvinyl alcohol, carboxymethylcellulose, starch and starch derivatives and copolymer emulsions of styrene/butadiene, vinyl acetate/ethylene, vinyl acetate/vinyl chloride/ethylene and methacrylate/butadiene are disclosed as suitable binders for the protective layer. Precipitated calcium carbonate, milled calcium carbonate, talc, kaolin, anhydrous silica, magnesium carbonate, zinc oxide, alumina and aluminum hydroxide are mentioned as suitable pigments in addition to organic pigments. Pigment-containing protective layers which contain only water-soluble binder are described by way of example.
EP-B-0 373 903 relates to a thermosensitive recording paper having an internal adhesive strength of 2.5 kg·cm or more, which has a protective layer having a smoothness of at least 500 Bekk seconds, preferably of from 700 to 1,500 Bekk seconds, which is applied to an intermediate layer containing fine particles of a styrene/acrylate polymer and a binder. Both the water-soluble and water-insoluble high polymers having adequate film formation capability are designated as binders suitable for the protective layer. An optional pigment addition to the protective layer is likewise disclosed, but the publication provides no information about the type of suitable pigments.
The known thermosensitive recording materials have proven useful in numerous applications, for example, as office papers and also as tickets, but new requirement profiles, which are often contradictory, constantly necessitate completely novel innovative products which break with the known approaches to a solution.
With the already known use of thermosensitive recording materials as a ticket, and in particular, as a travel coupon, there has been a growing need for materials which have high resistance to the environment. By this is meant that the thermosensitive recording material has an excellent resistance to water, plasticizers, oils and fats. The requirement for resistance to water arises from the fact that characters produced on a travel coupon by the supply of heat must remain decipherable and checkable for the duration of an interval of use even when the travel coupon cannot be protected by storage from exposure to the elements, such as on rainy days. In the same way, the travel coupon cannot be permitted to lose its characters on contact with fats and oils. Since travel coupons are frequently stored in document pockets in order to protect them from mechanical destruction, it is furthermore necessary for the thermosensitive recording materials used in this manner to have high resistance to plasticizers such as those usually contained in document pockets.
There is also a requirement for higher smudge resistance compared with the prior art, and hence more reliable cancelability which, in the context of the object of this invention, is to be understood as meaning that stamped-on cancellation marks cannot be wiped away after about 10 seconds, either in the dry state or in the wet state. Up to now, the only known thermosensitive recording materials are those in which the cancellation stamp ink applied to the protective layer cannot be wiped away completely from the protective layer either in the dry state or in the wet state, but the smudge resistance is still not completely satisfactory in that wiping leads to a recognizably more blurred printed image in comparison with the same printed image prior to wiping.
Since tickets and, in particular, travel coupons usually have imprints applied by means of flexographic printing processes or, in particular, by means of wet offset printing processes, printability over the entire surface of the protective layer in the areas where the cancellation imprint is not intended is a further requirement.
The necessity for recording materials suitable for use as travel coupons to be capable of being produced economically should not be underestimated. None of the thermosensitive recording materials known to date has been able to completely fulfill this spectrum of requirements.